1. Field of the Invention
The present invention relates to a technique which provides measurement of the position of an object from an image captured by a camera unit.
2. Discussion of Background Art
Three-dimensional measurement with images has recently been prevailing as a result of development of processing for the image of a CCD camera or a computer. There are examples such as a technique which measures the position of an object with plural cameras, and another one which uses a combination of a camera and a floodlight. The former calculates a distance from the camera to the object with triangulation incorporating a position of each camera and a pixel representative of the object in an image captured by each camera. On the other hand, the latter calculates it with triangulation based on a relative position between the camera and floodlight, and a position of a beam of collimated light in an image captured by the camera. Japanese Published Patent Application 9-287927 reports related arts associated with the latter technique. In this connection, these techniques are based on the assumption that a lens system of the camera can be modeled by a pin-hole camera. As shown in FIG. 1, a model of pin-hole camera is characterized in that only a beam of light coming through a pin-hole H reaches a plane of image, so that a three-dimensional space (x, y, z) is translated into a two-dimensional space (u, v). The model is thus generated on condition that the incident beam of light passes through the pin-hole, focusing an image in the camera.
However, it is known that the image captured by the camera with the lens system has intrinsic non-linear distortion, and the further it departs from the center, the larger the distortion will be. In this way, it is not possible to measure the accurate position of an object from the image having this type of distortion. A technique for improving the accuracy with compensation for the captured image has been proposed. Japanese Published Patent Application 5-274426 reports related arts.
The related arts described above, which make the compensation for the image based on the phenomenon that the peripheral distortion is larger, does not provide fundamental compensation. For example, selecting a matrix of feature points in the center of an image where the distortion is regarded as relatively limited, the technique disclosed by Japanese Published Patent Application 5-274426 calculates a reference point applied to the whole feature points of the image. Because the center is not free from the distortion, the reference point is accordingly calculated from the image having the distortion. In this way, although it may be possible for this technique to improve the accuracy for measurement of a position to some extent, there still remains a problem that the sufficiently accurate position measurement may not necessarily be implemented.
The conventional compensation for the distortion of lens system depends on a distance between the camera and object. If the distance is known, it may be possible to make compensation with a function of compensation for the distortion of image, which varies non-linearly according to the distance. However, it has not yet solved the problem because actually the distance is unknown before the measurement of a position is completed.